Introduce parent class for all FlickRaw exceptions

Sometimes I would like to rescue from any of exceptions related to flickraw. For this I have introduced a generic class `FlickRaw::Error` and inherit all other exceptions from it.

I have also created a new exception `FlickrAppNotConfigured` to raise when API key or secret is not set, instead of raising just plain RuntimeError.

Please advice if you do not like the wording (e.g.  `FlickRaw::Error` may be called  `FlickRaw::Exception`, or `FlickrAppNotConfigured` may be `ApiKeyOrSecretNotDefined`, or any other suggestions you may have)